Dehydrocyclodimerization (DHCD) is a process in which aliphatic hydrocarbons containing from 2 to 6 carbon atoms per molecule are reacted over a catalyst to produce a high yield of aromatics and hydrogen. This process is well known and is described in detail in U.S. Pat. Nos. 4,654,455 and 4,746,763 which are incorporated by reference. Typically, the dehydrocyclodimerization reaction is carried out at temperatures in excess of 500.degree. C., using dual functional catalysts containing acidic and dehydrogenation components. The acidic function is usually provided by a zeolite which promotes the oligomerization and aromatization reactions, while a non-noble metal component promotes the dehydrogenation function.
Since the product stream from the dehydrocyclodimerization process contains a mixture of compounds, it must undergo several separation steps in order to obtain usable products. A first fractionation zone will separate any unreacted feedstream from a C.sub.6.sup.+ product stream. The unreacted feedstream is recycled to the dehydrocyclodimerization zone while the product stream is fed to a second fractionation zone to separate the product mixture into benzene, toluene, xylenes and heavier aromatics.
Although the prior art states that C.sub.2 to C.sub.6 aliphatic hydrocarbons can be used in the dehydrocyclodimerization process, it also cautions that the amount of C.sub.5 and C.sub.6 hydrocarbons should be kept to a minimum, preferably less than 20 weight percent. The reason for this is that pentanes and hexanes are much more reactive than propane and butane at DHCD operating conditions, thereby forming considerable coke which quickly deactivates the catalyst. Another reason to minimize the amount of pentane in the feedstream is that pentane typically has a high economic value which would not be increased considerably by converting it to aromatics. However, there may be situations where it may be desirable to use a pentane/hexane feedstream to produce aromatic hydrocarbons. For example, C.sub.5 /C.sub.6 products in remote gas producing regions have a low economic value relative to aromatics. Since the current state of the art would not produce a high yield of aromatics, there is a need to develop a process which would provide a high yield of aromatics from a pentane/hexane feedstream.
Applicants have developed a process which solves this problem. A first step in applicants' process involves contacting the C.sub.5 /C.sub.6 feedstream with a DHCD catalyst at a temperature of about 300.degree. C. to about 500.degree. C. This operating temperature is about 50.degree. C. to 100.degree. C. lower than is used in a typical DHCD reactor. At this lower temperature the C.sub.5 /C.sub.6 hydrocarbons are converted to aromatics and C.sub.3 /C.sub.4 products with the majority of the products being propane and butane. Only small amounts of coke are formed at these lower temperatures. The C.sub.3 /C.sub.4 hydrocarbons are now contacted with a DHCD catalyst under normal DHCD conditions to produce aromatic products, hydrogen and byproducts. Finally, the aromatic products are separated in the usual manner.